High Voltage Testing - PowerWiki Engineering II - 9... · Single stage impulse generator U DC IG U imp Charging resistor R C Switching spark gap(U 0) ... impulse generator with utilization

Power Engineering II

High Voltage Testing

High Voltage Testing 2


HV Test Laboratories

• Voltage levels of transmission systems increase with the rise of transmitted power.

• Long-distance transmissions are often arranged by HVDC systems. However, a vast majority of transmission systems is still ensured by AC links.

• The purpose of HV test labs is to verify whether the insulation system can withstand the prescribed stresses.




• The insulation system is subjected to different types of electrical stress during the tests. These tests are:– Power frequency voltage withstand test

– Lightning impulse voltage withstand test

– Switching impulse voltage withstand test

• The amplitude of voltage and the form of voltage stress depend on both type and rated voltage of a tested machine.




• HV laboratories need to be equipped with relevant high voltage sources:

– High voltage DC power source

– High voltage AC power source

– High voltage impulse power source



DC High Voltage Power Sources

• Ripples (ripple factor) decrease as load RZ, capacity C or frequency of the voltage increase



AC High Voltage Power Sources

Grid switch

A

VPower Feed

Switch

A

V

RT

LT

CT

Grid 3x400 V

Test Transformer

Regulating Transformer

Compensation Reactor

Power Source

V



AC High Voltage Power Sources

• Better cooling conditions (can be equipped with radiators)

• Since the tank is grounded, the transformer can be placed next to other objects or walls.

• Viable construction for outdoor tests

• Does not require a bushing (the surface is insulated)

• Viable for employment as cascade transformers (up to 1500 MVA)

• Only usable indoors for short duration tests

Test Transformer with Insulated Cylindrical Tank

Test Transformer with Grounded Metallic Tank



Impulse High Voltage Power SourcesSingle stage impulse generator

UDC UIG Uimp

Charging resistor RC

Switching spark gap(U0)

Front resistor Rf

Tail resistor RtStorage capacitor C1

Load capacitor C2

UDCmax

Uimpp

Uimp

UIG

to tp

Impulse generator utilization factor

𝜂 =𝑈𝑖𝑚𝑝𝑝

𝑈𝐷𝐶𝑚𝑎𝑥< 1



Impulse High Voltage Power SourcesMultistage impulse generator

SG

SG

SG

SG

Ci

Ci

Ci

CiRc

Rf

Rt

Rf

Rt

Rf

Rt

Rf

Rt

Rc

Rc

Rc

3CL

3CL

3CL

Cm>>CL

Output for measuring devices

Grounding resistor and switch

DC voltage from power source

SGSA Impulse Generator Haefely Hipotronics

𝑈𝑖𝑚𝑝 = 𝑛𝜂𝑈0

Output voltage Uimp for an n-stage impulse generator with utilization factor η and source voltage U0:



Impulse Generator Triggering (Trigatron)

High voltage main electrode

Grounded main electrode

Triggering electrode

Connection to high voltage pulse generator (< 10kV)

High voltage pulse generator

Voltage comparator

The value of charging voltage UDC

Triggerable spark gap for output

impulse chopping (Chopping gap)

High voltage pulse generator

Digital recorder

Output from impulse voltage

divider



Output Voltage of Impulse Generator

• The polarity and peak voltage of impulse Um is determined from relevant standard for rated voltage of the tested machine

• Engineering tolerance of peak voltage is 3 %

• Front time T1=1,2 µs ± 30%• Tail time T2=50 µs ± 20%

• The polarity and peak voltage of impulse Um is determined from relevant standard for rated voltage of the tested machine

• Engineering tolerance of peak voltage is 3 %

• Front time Tv=250 µs ± 20 %• Tail time T2=2500 µs ± 60 %

Switching impulseLightning impulsefront tail



High Voltage Dividers

• Resistive, capacitive, inductive and combined high voltage dividers are used for high voltage measurements.

• Inductive dividers are primarily employed for calibration purposes as they are far too expensive for very high voltage measurements.

• The upper electrode of a divider is equipped with toroidal rings, which prevent generation of partial discharges.

• The current passing through the divider should be lower than 10 mA (from Ohm's law we can determine minimal resistance: 1 MΩ/10 kV)



Capacitive High Voltage Dividers

• Primarily used for AC high voltage measurements

• The high voltage part of the divider usually consists of several capacitors.

• The measured voltage U1 (HV) can be derived from output voltage U2 as:

𝑈1 = 𝑈2 1 +𝐶2𝐶1

• In practice, we must consider the effect of parasitic capacitances between the divider's cylinder and ground. Therefore, the previous relation must be modified.

C12




C11

C2Ce

d

Ie

I1

I2

𝐶𝑒 =2𝜋휀𝑙

𝑙𝑛2𝑙𝑑

4ℎ + 𝑙4ℎ + 3𝑙

l

h

When l>>d, Ce can be expressed approximately as:

U12

𝐼2 = 𝜔𝑈12𝐶12

𝐼𝑒 = 𝜔𝑈12𝐶𝑒

𝐼1 = 𝐼12 + 𝐼𝑒

The current flowing through C12 and C2 is:

The current flowing through parasitic capacitance Ce between shielding of the upper electrode and ground is:

By summing up both currents, we can calculate the total current flowing through the upper HV capacitor:

Voltage drop on the upper capacitor is then: 𝑈11 =𝐼11𝜔𝐶11




Capacitance Ce reduces the total capacity of the HV part of the divider. This effect is magnified with increasing number of employed capacitors.Effective capacity C1 of the HV part of the divider that consists of n capacitors of capacity C1n

can be expressed approximately as:

𝐶1 ≈𝐶1𝑛𝑛

−𝑛𝐶𝑒6

It was experimentally proven that capacity Ce of each capacitor is almost independent on its distance from ground. Therefore, we can utilize the simplified relation:

𝐶𝑒 =2𝜋휀𝑙

𝑙𝑛2𝑙𝑑

Hence

𝐶1 ≈𝐶1𝑛𝑛

−𝑛𝜋휀𝑙

3𝑙𝑛2𝑙𝑑



Resistive High Voltage Dividers

R12

R15

R1n

R14

R13

R11

mA

R12

R15

R1n

R14

R13

R11

I2 U2R2

• Used for DC or impulse measurements • The current through the divider should

not exceed 5 mA• Individual resistors are wrapped

around the insulation cylinder to form a helix



RC High Voltage Divider

U2

R1

R2

C1

C2U2

R1

R2

C1

C2

Parallel combination Series combination

• Parallel combination is used for AC, DC or impulse measurements, series combination is used only for AC or impulse measurements



Dynamic Behavior of HV Dividers• In the case of lightning impulse voltage tests, it is necessary to verify that

the dynamic behavior of a divider meets frequency range requirements. This is particularly important for impulses that are chopped in the tail part.

• Dynamic response of a divider to a unit step voltage is tested by a generator with rise time of units of ns and amplitude of hundreds of volts.

Normalized response of a divider to a unit step

𝑇𝑟𝑒𝑠 = න𝑂1

∞

1 − 𝑔 𝑡 𝑑𝑡 = 𝑇𝛼 − 𝑇𝛽 + 𝑇𝛾 −⋯

Response time of a divider:



Electrostatic Voltmeter• Direct method of high voltage AC and DC measurement• Electrostatic voltmeters can be directly connected to HV circuits of

voltages up to 200 kV. Should the voltage be any higher, voltage dividers must be employed.

Air/SF6/vacuum

Moving electrode with mirror

Fixed light source

Scale

Electric field energy density between electrodes:

𝑤𝑒 =1

2휀𝐸2

Energy density in element Sdx:

𝑑𝑤 = 𝑤𝑒𝑆𝑑𝑥 =1

2휀𝑆𝐸2𝑑𝑥

The force acting on the moving electrode:

𝐹 =𝑑𝑤

𝑑𝑥=1

2휀𝑆𝐸2 =

1

2휀𝑆𝑈2

𝑑2

Mean value of force of time variable voltage

1

𝑇න0

𝑇

𝐹 𝑡 𝑑𝑡 =휀𝑆

2𝑑2𝑇න0

𝑇

𝑈2 𝑡 =휀𝑆

2𝑑2𝑈𝑅𝑀𝑆2

d

dx

S



Spark Gaps• Breakdown voltage in homogeneous or almost homogeneous electric field, such as between two

sphere gaps in air, shows high stability and small deviation. Therefore, arrangements with homogeneous fields can be employed for peak voltage measurement.

• Nowadays, sphere gaps are not used for measurements on a daily basis. However, they are sometimes utilized for automatic measurement systems certification or linearity tests.

Dependence of breakdown voltage on electrode distance dfor different electrode diameters D



Adjustment to Actual Atmospheric Conditions

• The value of breakdown voltage measured on sphere gaps is determined from a table of normal atmospheric breakdown voltages. The results have to be adjusted to actual atmospheric conditions.

• Normal atmospheric conditions– Normal temperature tN = 20 °C– Normal pressure pN = 101,3 kPa– Normal absolute humidity gN = 11 g/m3

• Air density

𝛿 =𝑡𝑁 + 273

𝑡𝑎 + 273∙𝑝𝑎𝑝𝑁

• The real measured voltage Us can be calculated from table value Unas:

𝑈𝑆 = 𝑈𝑁𝑘ℎ𝑘𝑣

= 𝑈𝑁𝛿

• Where kh = δ for 0,95< δ <1,05 and kv= 1



Instrument Voltage Transformers• A special kind of transformers designed to transform high

voltage to low voltage (usually 100 V) with prescribedaccuracy

• Either inductive (<145 kV) or capacitive (>145 kV) construction.

Inductive instrument transformer Capacitive instrument transformer

ABB, Buyer's guide

1 – Primary terminal2 – Oil level sight glass3 – Oil4 – Quartz filling5 – Insulator

11 – Tank12 – Primary winding13 – Secondary windings14 – Core15 – Secondary terminals16 – Ground connection

6 – Lifting lug7 – Secondary terminal box8 – Neutral end terminal9 – Expansion system10 – Paper insulation

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High Voltage Testing - PowerWiki Engineering II - 9... · Single stage impulse generator U DC IG U imp Charging resistor R C Switching spark gap(U 0) ... impulse generator with utilization